Get the exact spreadsheet formula you need, or fix the broken one

Help me with a spreadsheet formula.

Spreadsheet app: [Google Sheets or Excel]
My columns: [e.g. A = date, B = customer, C = amount, D = paid yes/no]
What I want: [e.g. total of unpaid invoices older than 30 days]
Broken formula and error, if any: [paste it]
An example of the right answer: [e.g. with this data it should come to X]

First ask me up to 3 short questions if anything's unclear. Wait for my answers.

Then:
1. The exact formula and the cell to paste it in.
2. What each part does, in plain words.
3. A quick test with 3 rows so I can check it's right.
4. The most likely way it breaks later and how to avoid it.

Only use what I've told you, leave blanks for anything I haven't given, and don't invent facts or figures. UK English.

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